1. Field of the Invention
The present invention relates to electrophotography apparatuses, such as printers, copy machines, and facsimiles.
2. Description of the Related Art
As a result of the growing demand for producing documents in color at high speed, color printers are becoming increasingly common. For example, a color electrophotography apparatus is known in which a black toner and toners for the colors yellow, magenta, and cyan are used. Toner images formed by image forming units for the individual colors are transferred onto an intermediate transfer member, and a resultant toner image with the overlaid colors is transferred to and then fused on a recording medium, thereby obtaining a color image.
In this type of electrophotography apparatus, in order to obtain stable image quality in terms of image density and the like, image forming conditions are controlled by forming a plurality of testing solid patches on the intermediate transfer member under predetermined image forming conditions, and the amounts of toner attached in the patches are detected by an optical sensor.
Patent Documents 1 and 2 disclose methods for measuring the attached toner amounts. When measuring the amount of attached black toner, which absorbs light well and produces little scattered light, a method is used that utilizes a specular reflection output (Vreg) of a photoreceiving element on which specular reflection light is incident.
This method, however, is not suitable for measuring the attached amounts of color toners because the color toners produce much scattering of light and, as the attached toner amount increases, a scattered light component in the specular reflection output Vreg increases. Thus, a method is employed that uses an additional photoreceiving element on which diffusive reflected light alone is incident. In this method, a diffusive reflection output (Vdif) is measured simultaneously, and the scattered light component contained in the specular reflection output Vreg is removed on the basis of the diffusive reflection output.
Nevertheless, even with the use of the specular reflection output Vreg from which the scattered light component is removed as discussed in Patent Document 1, the upper limit of the measurable range of attached toner amount is no more than approximately one full layer of toner. Above that, the specular reflection output Vreg saturates and cannot be measured. Normally, the attached toner amount of a solid image that is set in an actual printing operation is in the saturation region and cannot be measured. Thus, a method is used by which large attached amounts outside the measurable range are estimated from a measurable low range of attached amount in view of the development characteristics and the like.
With regard to the measurement of the attached amounts of color toners, the diffusive reflection output Vdif may be corrected with reference to attached toner amount data in a low attached-amount range that can be measured by the specular reflection output Vreg. Then an attached toner amount may be calculated from the corrected diffusive reflection output, using an attached toner amount conversion table for diffusive reflection. In this way, the high-density attached amounts in solid images can be determined.
There are two kinds of the testing toner patches that are conventionally used: one is a solid patch formed by solid exposure; and the other is a halftone patch for which exposure is turned on and off repeatedly in order to form a halftone image, such as a halftone dot image.
The solid patch is used for controlling the attached toner amount in a solid image region within a recorded image. For example, a number of the solid patches are formed while varying the developing bias potential as an image forming condition, and their attached toner amounts are measured with an optical sensor. In this way, a developing bias potential for obtaining a desired attached amount for a solid image can be determined.
On the other hand, the halftone patch is used for controlling the attached toner amount in a halftone dot or grey level image region within a recorded image. For example, multiple halftone patches are formed while varying a laser output as an image forming condition, and their attached toner amounts are measured with an optical sensor. In this way, a laser output for obtaining a desired attached toner amount can be determined.
The size of such testing toner patches is normally on the order of 10 mm×10 mm. The attached toner amount in an edge region within 0.3 to 0.6 mm of the image edge is typically larger than the attached toner amount in the inner region of the testing patch. This is due to a long-known phenomenon referred to as a fringing field effect, or the edge effect.
In accordance with the related art disclosed in Patent Documents 1 and 2, only the inner, central region of the testing toner patch is measured and controlled, so that the attached toner amount in the aforementioned edge region cannot be controlled to a desired value (which is normally the same as the attached toner amount in the inner region). This problem has been overcome by the related art as follows.
Patent Documents 3 and 4 disclose that a halftone patch is formed, and the amount of attached (developed) toner in the image edge portion is measured. The edge portions of a halftone dot image, a thin line image, and a solid image are recognized by pattern recognition technology, and the amount of exposure or the like is selectively changed within the image in order to reduce the edge effect. Patent Document 5 discloses that, after measuring an attached toner amount, the exposure amount or the like is modulated using a spatial digital filter instead of pattern recognition technology, so that the attached toner amount within the image edge portion can be corrected.    Patent Document 1: Japanese Laid-Open Patent Application No. 2005-77685    Patent Document 2: Japanese Laid-Open Patent Application No. 2002-236402    Patent Document 3: Japanese Laid-Open Patent Application No. 2003-98773    Patent Document 4: Japanese Patent No. 3479447    Patent Document 5: Japanese Patent No. 3373556
When the technologies according to Patent Documents 3 and 4 are applied to a high-speed electrophotography apparatus, the following problems arise.
First, a single-dot image or a line with a single-dot width either becomes blurred or may not be recorded at all. This is because, although the electric field intensity tends to enhance the edges during development due to the edge effect, this does not necessarily result in a corresponding amount of toner that is developed. Rather, in a high-speed machine, the attached toner amount in a region up to about 0.1 mm from the image edge is smaller than in the central portion. The attached toner amount increases from the aforementioned region and reaches a maximum (peak) attached toner amount at around 0.2 mm from the image edge. The attached toner amount then decreases further within, until it becomes the same as the attached toner amount at the central portion.
It goes without saying that the peak position or amount of attached toner differs among the edge portions upstream, downstream, and at the sides of the patch. Thus, when the conventional art is used, what little small amount of attached toner of a single-dot image or a single-dot-width line decreases even more, resulting in a blurred image or no image at all.
Another problem is that it is difficult with high-speed machines to accurately control the amount of exposure from a laser light source in multiple levels. This is due to the fact that their laser modulating speed is too fast. Thus, in the case of a high-speed apparatus, appropriate exposure intensities cannot be set for the upstream, downstream, side, and 45°-inclined edge portions of a solid image individually as shown in FIG. 7 of Patent Document 4. Further, exposure intensity cannot be accurately modulated based on an output of a digital filter as disclosed in Patent Document 5; the conventional exposure intensity may be reduced stably by only one level.
Because a halftone dot image is normally highly accurately density-controlled in a gradation process by an upper-level controller, image quality may deteriorate if the exposure intensity for an edge portion of the halftone dot image is inaccurately modulated. Thus, the edge control for halftone dot images should be left to the gradation process by the upper-level controller, and the edges of solid images alone should be corrected using the conventional art.